Create a Message

post /v1/messages

Send a structured list of input messages with text and/or image content, and the model will generate the next message in the conversation.

The Messages API can be used for either single queries or stateless multi-turn conversations.

Learn more about the Messages API in our user guide

Body Parameters

• max_tokens: number

  The maximum number of tokens to generate before stopping.

  Note that our models may stop before reaching this maximum. This parameter only specifies the absolute maximum number of tokens to generate.

  Set to 0 to populate the prompt cache without generating a response.

  Different models have different maximum values for this parameter. See models for details.

• messages: array of MessageParam

  Input messages.

  Our models are trained to operate on alternating user and assistant conversational turns. When creating a new Message, you specify the prior conversational turns with the messages parameter, and the model then generates the next Message in the conversation. Consecutive user or assistant turns in your request will be combined into a single turn.

  Each input message must be an object with a role and content. You can specify a single user-role message, or you can include multiple user and assistant messages.

  If the final message uses the assistant role, the response content will continue immediately from the content in that message. This can be used to constrain part of the model's response.

  Example with a single user message:

  [{"role": "user", "content": "Hello, Claude"}]
  

  Example with multiple conversational turns:

  [
    {"role": "user", "content": "Hello there."},
    {"role": "assistant", "content": "Hi, I'm Claude. How can I help you?"},
    {"role": "user", "content": "Can you explain LLMs in plain English?"},
  ]
  

  Example with a partially-filled response from Claude:

  [
    {"role": "user", "content": "What's the Greek name for Sun? (A) Sol (B) Helios (C) Sun"},
    {"role": "assistant", "content": "The best answer is ("},
  ]
  

  Each input message content may be either a single string or an array of content blocks, where each block has a specific type. Using a string for content is shorthand for an array of one content block of type "text". The following input messages are equivalent:

  {"role": "user", "content": "Hello, Claude"}
  

  {"role": "user", "content": [{"type": "text", "text": "Hello, Claude"}]}
  

  See input examples.

  Note that if you want to include a system prompt, you can use the top-level system parameter — there is no "system" role for input messages in the Messages API.

  There is a limit of 100,000 messages in a single request.

  • content: string or array of ContentBlockParam

    • string

    • array of ContentBlockParam

      • TextBlockParam object { text, type, cache_control, citations }

        • text: string

        • type: "text"

          • "text"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

          • type: "ephemeral"

            • "ephemeral"

          • ttl: optional "5m" or "1h"

            The time-to-live for the cache control breakpoint.

            This may be one the following values:

            • 5m: 5 minutes
            • 1h: 1 hour

            Defaults to 5m.

            • "5m"

            • "1h"

        • citations: optional array of TextCitationParam

          • CitationCharLocationParam object { cited_text, document_index, document_title, 3 more }

            • cited_text: string

            • document_index: number

            • document_title: string

            • end_char_index: number

            • start_char_index: number

            • type: "char_location"

              • "char_location"

          • CitationPageLocationParam object { cited_text, document_index, document_title, 3 more }

            • cited_text: string

            • document_index: number

            • document_title: string

            • end_page_number: number

            • start_page_number: number

            • type: "page_location"

              • "page_location"

          • CitationContentBlockLocationParam object { cited_text, document_index, document_title, 3 more }

            • cited_text: string

              The full text of the cited block range, concatenated.

              Always equals the contents of content[start_block_index:end_block_index] joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

            • document_index: number

            • document_title: string

            • end_block_index: number

              Exclusive 0-based end index of the cited block range in the source's content array.

              Always greater than start_block_index; a single-block citation has end_block_index = start_block_index + 1.

            • start_block_index: number

              0-based index of the first cited block in the source's content array.

            • type: "content_block_location"

              • "content_block_location"

          • CitationWebSearchResultLocationParam object { cited_text, encrypted_index, title, 2 more }

            • cited_text: string

            • encrypted_index: string

            • title: string

            • type: "web_search_result_location"

              • "web_search_result_location"

            • url: string

          • CitationSearchResultLocationParam object { cited_text, end_block_index, search_result_index, 4 more }

            • cited_text: string

              The full text of the cited block range, concatenated.

              Always equals the contents of content[start_block_index:end_block_index] joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

            • end_block_index: number

              Exclusive 0-based end index of the cited block range in the source's content array.

              Always greater than start_block_index; a single-block citation has end_block_index = start_block_index + 1.

            • search_result_index: number

              0-based index of the cited search result among all search_result content blocks in the request, in the order they appear across messages and tool results.

              Counted separately from document_index; server-side web search results are not included in this count.

            • source: string

            • start_block_index: number

              0-based index of the first cited block in the source's content array.

            • title: string

            • type: "search_result_location"

              • "search_result_location"

      • ImageBlockParam object { source, type, cache_control }

        • source: Base64ImageSource or URLImageSource

          • Base64ImageSource object { data, media_type, type }

            • data: string

            • media_type: "image/jpeg" or "image/png" or "image/gif" or "image/webp"

              • "image/jpeg"

              • "image/png"

              • "image/gif"

              • "image/webp"

            • type: "base64"

              • "base64"

          • URLImageSource object { type, url }

            • type: "url"

              • "url"

            • url: string

        • type: "image"

          • "image"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

      • DocumentBlockParam object { source, type, cache_control, 3 more }

        • source: Base64PDFSource or PlainTextSource or ContentBlockSource or URLPDFSource

          • Base64PDFSource object { data, media_type, type }

            • data: string

            • media_type: "application/pdf"

              • "application/pdf"

            • type: "base64"

              • "base64"

          • PlainTextSource object { data, media_type, type }

            • data: string

            • media_type: "text/plain"

              • "text/plain"

            • type: "text"

              • "text"

          • ContentBlockSource object { content, type }

            • content: string or array of ContentBlockSourceContent

              • string

              • ContentBlockSourceContent = array of ContentBlockSourceContent

                • TextBlockParam object { text, type, cache_control, citations }

                • ImageBlockParam object { source, type, cache_control }

            • type: "content"

              • "content"

          • URLPDFSource object { type, url }

            • type: "url"

              • "url"

            • url: string

        • type: "document"

          • "document"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • citations: optional CitationsConfigParam

          • enabled: optional boolean

        • context: optional string

        • title: optional string

      • SearchResultBlockParam object { content, source, title, 3 more }

        • content: array of TextBlockParam

          • text: string

          • type: "text"

          • cache_control: optional CacheControlEphemeral

            Create a cache control breakpoint at this content block.

          • citations: optional array of TextCitationParam

        • source: string

        • title: string

        • type: "search_result"

          • "search_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • citations: optional CitationsConfigParam

      • ThinkingBlockParam object { signature, thinking, type }

        • signature: string

        • thinking: string

        • type: "thinking"

          • "thinking"

      • RedactedThinkingBlockParam object { data, type }

        • data: string

        • type: "redacted_thinking"

          • "redacted_thinking"

      • ToolUseBlockParam object { id, input, name, 3 more }

        • id: string

        • input: map[unknown]

        • name: string

        • type: "tool_use"

          • "tool_use"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • caller: optional DirectCaller or ServerToolCaller or ServerToolCaller20260120

          Tool invocation directly from the model.

          • DirectCaller object { type }

            Tool invocation directly from the model.

            • type: "direct"

              • "direct"

          • ServerToolCaller object { tool_id, type }

            Tool invocation generated by a server-side tool.

            • tool_id: string

            • type: "code_execution_20250825"

              • "code_execution_20250825"

          • ServerToolCaller20260120 object { tool_id, type }

            • tool_id: string

            • type: "code_execution_20260120"

              • "code_execution_20260120"

      • ToolResultBlockParam object { tool_use_id, type, cache_control, 2 more }

        • tool_use_id: string

        • type: "tool_result"

          • "tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • content: optional string or array of TextBlockParam or ImageBlockParam or SearchResultBlockParam or 2 more

          • string

          • array of TextBlockParam or ImageBlockParam or SearchResultBlockParam or 2 more

            • TextBlockParam object { text, type, cache_control, citations }

            • ImageBlockParam object { source, type, cache_control }

            • SearchResultBlockParam object { content, source, title, 3 more }

            • DocumentBlockParam object { source, type, cache_control, 3 more }

            • ToolReferenceBlockParam object { tool_name, type, cache_control }

              Tool reference block that can be included in tool_result content.

              • tool_name: string

              • type: "tool_reference"

                • "tool_reference"

              • cache_control: optional CacheControlEphemeral

                Create a cache control breakpoint at this content block.

        • is_error: optional boolean

      • ServerToolUseBlockParam object { id, input, name, 3 more }

        • id: string

        • input: map[unknown]

        • name: "web_search" or "web_fetch" or "code_execution" or 4 more

          • "web_search"

          • "web_fetch"

          • "code_execution"

          • "bash_code_execution"

          • "text_editor_code_execution"

          • "tool_search_tool_regex"

          • "tool_search_tool_bm25"

        • type: "server_tool_use"

          • "server_tool_use"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • caller: optional DirectCaller or ServerToolCaller or ServerToolCaller20260120

          Tool invocation directly from the model.

          • DirectCaller object { type }

            Tool invocation directly from the model.

          • ServerToolCaller object { tool_id, type }

            Tool invocation generated by a server-side tool.

          • ServerToolCaller20260120 object { tool_id, type }

      • WebSearchToolResultBlockParam object { content, tool_use_id, type, 2 more }

        • content: WebSearchToolResultBlockParamContent

          • WebSearchToolResultBlockItem = array of WebSearchResultBlockParam

            • encrypted_content: string

            • title: string

            • type: "web_search_result"

              • "web_search_result"

            • url: string

            • page_age: optional string

          • WebSearchToolRequestError object { error_code, type }

            • error_code: WebSearchToolResultErrorCode

              • "invalid_tool_input"

              • "unavailable"

              • "max_uses_exceeded"

              • "too_many_requests"

              • "query_too_long"

              • "request_too_large"

            • type: "web_search_tool_result_error"

              • "web_search_tool_result_error"

        • tool_use_id: string

        • type: "web_search_tool_result"

          • "web_search_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • caller: optional DirectCaller or ServerToolCaller or ServerToolCaller20260120

          Tool invocation directly from the model.

          • DirectCaller object { type }

            Tool invocation directly from the model.

          • ServerToolCaller object { tool_id, type }

            Tool invocation generated by a server-side tool.

          • ServerToolCaller20260120 object { tool_id, type }

      • WebFetchToolResultBlockParam object { content, tool_use_id, type, 2 more }

        • content: WebFetchToolResultErrorBlockParam or WebFetchBlockParam

          • WebFetchToolResultErrorBlockParam object { error_code, type }

            • error_code: WebFetchToolResultErrorCode

              • "invalid_tool_input"

              • "url_too_long"

              • "url_not_allowed"

              • "url_not_accessible"

              • "unsupported_content_type"

              • "too_many_requests"

              • "max_uses_exceeded"

              • "unavailable"

            • type: "web_fetch_tool_result_error"

              • "web_fetch_tool_result_error"

          • WebFetchBlockParam object { content, type, url, retrieved_at }

            • content: DocumentBlockParam

            • type: "web_fetch_result"

              • "web_fetch_result"

            • url: string

              Fetched content URL

            • retrieved_at: optional string

              ISO 8601 timestamp when the content was retrieved

        • tool_use_id: string

        • type: "web_fetch_tool_result"

          • "web_fetch_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

        • caller: optional DirectCaller or ServerToolCaller or ServerToolCaller20260120

          Tool invocation directly from the model.

          • DirectCaller object { type }

            Tool invocation directly from the model.

          • ServerToolCaller object { tool_id, type }

            Tool invocation generated by a server-side tool.

          • ServerToolCaller20260120 object { tool_id, type }

      • CodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }

        • content: CodeExecutionToolResultBlockParamContent

          Code execution result with encrypted stdout for PFC + web_search results.

          • CodeExecutionToolResultErrorParam object { error_code, type }

            • error_code: CodeExecutionToolResultErrorCode

              • "invalid_tool_input"

              • "unavailable"

              • "too_many_requests"

              • "execution_time_exceeded"

            • type: "code_execution_tool_result_error"

              • "code_execution_tool_result_error"

          • CodeExecutionResultBlockParam object { content, return_code, stderr, 2 more }

            • content: array of CodeExecutionOutputBlockParam

              • file_id: string

              • type: "code_execution_output"

                • "code_execution_output"

            • return_code: number

            • stderr: string

            • stdout: string

            • type: "code_execution_result"

              • "code_execution_result"

          • EncryptedCodeExecutionResultBlockParam object { content, encrypted_stdout, return_code, 2 more }

            Code execution result with encrypted stdout for PFC + web_search results.

            • content: array of CodeExecutionOutputBlockParam

              • file_id: string

              • type: "code_execution_output"

            • encrypted_stdout: string

            • return_code: number

            • stderr: string

            • type: "encrypted_code_execution_result"

              • "encrypted_code_execution_result"

        • tool_use_id: string

        • type: "code_execution_tool_result"

          • "code_execution_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

      • BashCodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }

        • content: BashCodeExecutionToolResultErrorParam or BashCodeExecutionResultBlockParam

          • BashCodeExecutionToolResultErrorParam object { error_code, type }

            • error_code: BashCodeExecutionToolResultErrorCode

              • "invalid_tool_input"

              • "unavailable"

              • "too_many_requests"

              • "execution_time_exceeded"

              • "output_file_too_large"

            • type: "bash_code_execution_tool_result_error"

              • "bash_code_execution_tool_result_error"

          • BashCodeExecutionResultBlockParam object { content, return_code, stderr, 2 more }

            • content: array of BashCodeExecutionOutputBlockParam

              • file_id: string

              • type: "bash_code_execution_output"

                • "bash_code_execution_output"

            • return_code: number

            • stderr: string

            • stdout: string

            • type: "bash_code_execution_result"

              • "bash_code_execution_result"

        • tool_use_id: string

        • type: "bash_code_execution_tool_result"

          • "bash_code_execution_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

      • TextEditorCodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }

        • content: TextEditorCodeExecutionToolResultErrorParam or TextEditorCodeExecutionViewResultBlockParam or TextEditorCodeExecutionCreateResultBlockParam or TextEditorCodeExecutionStrReplaceResultBlockParam

          • TextEditorCodeExecutionToolResultErrorParam object { error_code, type, error_message }

            • error_code: TextEditorCodeExecutionToolResultErrorCode

              • "invalid_tool_input"

              • "unavailable"

              • "too_many_requests"

              • "execution_time_exceeded"

              • "file_not_found"

            • type: "text_editor_code_execution_tool_result_error"

              • "text_editor_code_execution_tool_result_error"

            • error_message: optional string

          • TextEditorCodeExecutionViewResultBlockParam object { content, file_type, type, 3 more }

            • content: string

            • file_type: "text" or "image" or "pdf"

              • "text"

              • "image"

              • "pdf"

            • type: "text_editor_code_execution_view_result"

              • "text_editor_code_execution_view_result"

            • num_lines: optional number

            • start_line: optional number

            • total_lines: optional number

          • TextEditorCodeExecutionCreateResultBlockParam object { is_file_update, type }

            • is_file_update: boolean

            • type: "text_editor_code_execution_create_result"

              • "text_editor_code_execution_create_result"

          • TextEditorCodeExecutionStrReplaceResultBlockParam object { type, lines, new_lines, 3 more }

            • type: "text_editor_code_execution_str_replace_result"

              • "text_editor_code_execution_str_replace_result"

            • lines: optional array of string

            • new_lines: optional number

            • new_start: optional number

            • old_lines: optional number

            • old_start: optional number

        • tool_use_id: string

        • type: "text_editor_code_execution_tool_result"

          • "text_editor_code_execution_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

      • ToolSearchToolResultBlockParam object { content, tool_use_id, type, cache_control }

        • content: ToolSearchToolResultErrorParam or ToolSearchToolSearchResultBlockParam

          • ToolSearchToolResultErrorParam object { error_code, type }

            • error_code: ToolSearchToolResultErrorCode

              • "invalid_tool_input"

              • "unavailable"

              • "too_many_requests"

              • "execution_time_exceeded"

            • type: "tool_search_tool_result_error"

              • "tool_search_tool_result_error"

          • ToolSearchToolSearchResultBlockParam object { tool_references, type }

            • tool_references: array of ToolReferenceBlockParam

              • tool_name: string

              • type: "tool_reference"

              • cache_control: optional CacheControlEphemeral

                Create a cache control breakpoint at this content block.

            • type: "tool_search_tool_search_result"

              • "tool_search_tool_search_result"

        • tool_use_id: string

        • type: "tool_search_tool_result"

          • "tool_search_tool_result"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

      • ContainerUploadBlockParam object { file_id, type, cache_control }

        A content block that represents a file to be uploaded to the container Files uploaded via this block will be available in the container's input directory.

        • file_id: string

        • type: "container_upload"

          • "container_upload"

        • cache_control: optional CacheControlEphemeral

          Create a cache control breakpoint at this content block.

  • role: "user" or "assistant"

    • "user"

    • "assistant"

• model: Model

  The model that will complete your prompt.

  See models for additional details and options.

  • "claude-opus-4-7" or "claude-mythos-preview" or "claude-opus-4-6" or 14 more

    The model that will complete your prompt.

    See models for additional details and options.

    • "claude-opus-4-7"

      Frontier intelligence for long-running agents and coding

    • "claude-mythos-preview"

      New class of intelligence, strongest in coding and cybersecurity

    • "claude-opus-4-6"

      Frontier intelligence for long-running agents and coding

    • "claude-sonnet-4-6"

      Best combination of speed and intelligence

    • "claude-haiku-4-5"

      Fastest model with near-frontier intelligence

    • "claude-haiku-4-5-20251001"

      Fastest model with near-frontier intelligence

    • "claude-opus-4-5"

      Premium model combining maximum intelligence with practical performance

    • "claude-opus-4-5-20251101"

      Premium model combining maximum intelligence with practical performance

    • "claude-sonnet-4-5"

      High-performance model for agents and coding

    • "claude-sonnet-4-5-20250929"

      High-performance model for agents and coding

    • "claude-opus-4-1"

      Exceptional model for specialized complex tasks

    • "claude-opus-4-1-20250805"

      Exceptional model for specialized complex tasks

    • "claude-opus-4-0"

      Powerful model for complex tasks

    • "claude-opus-4-20250514"

      Powerful model for complex tasks

    • "claude-sonnet-4-0"

      High-performance model with extended thinking

    • "claude-sonnet-4-20250514"

      High-performance model with extended thinking

    • "claude-3-haiku-20240307"

      Fast and cost-effective model

  • string

• cache_control: optional CacheControlEphemeral

  Top-level cache control automatically applies a cache_control marker to the last cacheable block in the request.

• container: optional string

  Container identifier for reuse across requests.

• inference_geo: optional string

  Specifies the geographic region for inference processing. If not specified, the workspace's default_inference_geo is used.

• metadata: optional Metadata

  An object describing metadata about the request.

  • user_id: optional string

    An external identifier for the user who is associated with the request.

    This should be a uuid, hash value, or other opaque identifier. Anthropic may use this id to help detect abuse. Do not include any identifying information such as name, email address, or phone number.

• output_config: optional OutputConfig

  Configuration options for the model's output, such as the output format.

  • effort: optional "low" or "medium" or "high" or 2 more

    All possible effort levels.

    • "low"

    • "medium"

    • "high"

    • "xhigh"

    • "max"

  • format: optional JSONOutputFormat

    A schema to specify Claude's output format in responses. See structured outputs

    • schema: map[unknown]

      The JSON schema of the format

    • type: "json_schema"

      • "json_schema"

• service_tier: optional "auto" or "standard_only"

  Determines whether to use priority capacity (if available) or standard capacity for this request.

  Anthropic offers different levels of service for your API requests. See service-tiers for details.

  • "auto"

  • "standard_only"

• stop_sequences: optional array of string

  Custom text sequences that will cause the model to stop generating.

  Our models will normally stop when they have naturally completed their turn, which will result in a response stop_reason of "end_turn".

  If you want the model to stop generating when it encounters custom strings of text, you can use the stop_sequences parameter. If the model encounters one of the custom sequences, the response stop_reason value will be "stop_sequence" and the response stop_sequence value will contain the matched stop sequence.

• stream: optional boolean

  Whether to incrementally stream the response using server-sent events.

  See streaming for details.

• system: optional string or array of TextBlockParam

  System prompt.

  A system prompt is a way of providing context and instructions to Claude, such as specifying a particular goal or role. See our guide to system prompts.

  • string

  • array of TextBlockParam

    • text: string

    • type: "text"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • citations: optional array of TextCitationParam

• temperature: optional number

  Amount of randomness injected into the response.

  Defaults to 1.0. Ranges from 0.0 to 1.0. Use temperature closer to 0.0 for analytical / multiple choice, and closer to 1.0 for creative and generative tasks.

  Note that even with temperature of 0.0, the results will not be fully deterministic.

• thinking: optional ThinkingConfigParam

  Configuration for enabling Claude's extended thinking.

  When enabled, responses include thinking content blocks showing Claude's thinking process before the final answer. Requires a minimum budget of 1,024 tokens and counts towards your max_tokens limit.

  See extended thinking for details.

  • ThinkingConfigEnabled object { budget_tokens, type, display }

    • budget_tokens: number

      Determines how many tokens Claude can use for its internal reasoning process. Larger budgets can enable more thorough analysis for complex problems, improving response quality.

      Must be ≥1024 and less than max_tokens.

      See extended thinking for details.

    • type: "enabled"

      • "enabled"

    • display: optional "summarized" or "omitted"

      Controls how thinking content appears in the response. When set to summarized, thinking is returned normally. When set to omitted, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to summarized.

      • "summarized"

      • "omitted"

  • ThinkingConfigDisabled object { type }

    • type: "disabled"

      • "disabled"

  • ThinkingConfigAdaptive object { type, display }

    • type: "adaptive"

      • "adaptive"

    • display: optional "summarized" or "omitted"

      Controls how thinking content appears in the response. When set to summarized, thinking is returned normally. When set to omitted, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to summarized.

      • "summarized"

      • "omitted"

• tool_choice: optional ToolChoice

  How the model should use the provided tools. The model can use a specific tool, any available tool, decide by itself, or not use tools at all.

  • ToolChoiceAuto object { type, disable_parallel_tool_use }

    The model will automatically decide whether to use tools.

    • type: "auto"

      • "auto"

    • disable_parallel_tool_use: optional boolean

      Whether to disable parallel tool use.

      Defaults to false. If set to true, the model will output at most one tool use.

  • ToolChoiceAny object { type, disable_parallel_tool_use }

    The model will use any available tools.

    • type: "any"

      • "any"

    • disable_parallel_tool_use: optional boolean

      Whether to disable parallel tool use.

      Defaults to false. If set to true, the model will output exactly one tool use.

  • ToolChoiceTool object { name, type, disable_parallel_tool_use }

    The model will use the specified tool with tool_choice.name.

    • name: string

      The name of the tool to use.

    • type: "tool"

      • "tool"

    • disable_parallel_tool_use: optional boolean

      Whether to disable parallel tool use.

      Defaults to false. If set to true, the model will output exactly one tool use.

  • ToolChoiceNone object { type }

    The model will not be allowed to use tools.

    • type: "none"

      • "none"

• tools: optional array of ToolUnion

  Definitions of tools that the model may use.

  If you include tools in your API request, the model may return tool_use content blocks that represent the model's use of those tools. You can then run those tools using the tool input generated by the model and then optionally return results back to the model using tool_result content blocks.

  There are two types of tools: client tools and server tools. The behavior described below applies to client tools. For server tools, see their individual documentation as each has its own behavior (e.g., the web search tool).

  Each tool definition includes:

  • name: Name of the tool.
  • description: Optional, but strongly-recommended description of the tool.
  • input_schema: JSON schema for the tool input shape that the model will produce in tool_use output content blocks.

  For example, if you defined tools as:

  [
    {
      "name": "get_stock_price",
      "description": "Get the current stock price for a given ticker symbol.",
      "input_schema": {
        "type": "object",
        "properties": {
          "ticker": {
            "type": "string",
            "description": "The stock ticker symbol, e.g. AAPL for Apple Inc."
          }
        },
        "required": ["ticker"]
      }
    }
  ]
  

  And then asked the model "What's the S&P 500 at today?", the model might produce tool_use content blocks in the response like this:

  [
    {
      "type": "tool_use",
      "id": "toolu_01D7FLrfh4GYq7yT1ULFeyMV",
      "name": "get_stock_price",
      "input": { "ticker": "^GSPC" }
    }
  ]
  

  You might then run your get_stock_price tool with {"ticker": "^GSPC"} as an input, and return the following back to the model in a subsequent user message:

  [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01D7FLrfh4GYq7yT1ULFeyMV",
      "content": "259.75 USD"
    }
  ]
  

  Tools can be used for workflows that include running client-side tools and functions, or more generally whenever you want the model to produce a particular JSON structure of output.

  See our guide for more details.

  • Tool object { input_schema, name, allowed_callers, 7 more }

    • input_schema: object { type, properties, required }

      JSON schema for this tool's input.

      This defines the shape of the input that your tool accepts and that the model will produce.

      • type: "object"

        • "object"

      • properties: optional map[unknown]

      • required: optional array of string

    • name: string

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • description: optional string

      Description of what this tool does.

      Tool descriptions should be as detailed as possible. The more information that the model has about what the tool is and how to use it, the better it will perform. You can use natural language descriptions to reinforce important aspects of the tool input JSON schema.

    • eager_input_streaming: optional boolean

      Enable eager input streaming for this tool. When true, tool input parameters will be streamed incrementally as they are generated, and types will be inferred on-the-fly rather than buffering the full JSON output. When false, streaming is disabled for this tool even if the fine-grained-tool-streaming beta is active. When null (default), uses the default behavior based on beta headers.

    • input_examples: optional array of map[unknown]

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

    • type: optional "custom"

      • "custom"

  • ToolBash20250124 object { name, type, allowed_callers, 4 more }

    • name: "bash"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "bash"

    • type: "bash_20250124"

      • "bash_20250124"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • input_examples: optional array of map[unknown]

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • CodeExecutionTool20250522 object { name, type, allowed_callers, 3 more }

    • name: "code_execution"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "code_execution"

    • type: "code_execution_20250522"

      • "code_execution_20250522"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • CodeExecutionTool20250825 object { name, type, allowed_callers, 3 more }

    • name: "code_execution"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "code_execution"

    • type: "code_execution_20250825"

      • "code_execution_20250825"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • CodeExecutionTool20260120 object { name, type, allowed_callers, 3 more }

    Code execution tool with REPL state persistence (daemon mode + gVisor checkpoint).

    • name: "code_execution"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "code_execution"

    • type: "code_execution_20260120"

      • "code_execution_20260120"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • MemoryTool20250818 object { name, type, allowed_callers, 4 more }

    • name: "memory"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "memory"

    • type: "memory_20250818"

      • "memory_20250818"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • input_examples: optional array of map[unknown]

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • ToolTextEditor20250124 object { name, type, allowed_callers, 4 more }

    • name: "str_replace_editor"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "str_replace_editor"

    • type: "text_editor_20250124"

      • "text_editor_20250124"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • input_examples: optional array of map[unknown]

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • ToolTextEditor20250429 object { name, type, allowed_callers, 4 more }

    • name: "str_replace_based_edit_tool"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "str_replace_based_edit_tool"

    • type: "text_editor_20250429"

      • "text_editor_20250429"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • input_examples: optional array of map[unknown]

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • ToolTextEditor20250728 object { name, type, allowed_callers, 5 more }

    • name: "str_replace_based_edit_tool"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "str_replace_based_edit_tool"

    • type: "text_editor_20250728"

      • "text_editor_20250728"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • input_examples: optional array of map[unknown]

    • max_characters: optional number

      Maximum number of characters to display when viewing a file. If not specified, defaults to displaying the full file.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • WebSearchTool20250305 object { name, type, allowed_callers, 7 more }

    • name: "web_search"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "web_search"

    • type: "web_search_20250305"

      • "web_search_20250305"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • allowed_domains: optional array of string

      If provided, only these domains will be included in results. Cannot be used alongside blocked_domains.

    • blocked_domains: optional array of string

      If provided, these domains will never appear in results. Cannot be used alongside allowed_domains.

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • max_uses: optional number

      Maximum number of times the tool can be used in the API request.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

    • user_location: optional UserLocation

      Parameters for the user's location. Used to provide more relevant search results.

      • type: "approximate"

        • "approximate"

      • city: optional string

        The city of the user.

      • country: optional string

        The two letter ISO country code of the user.

      • region: optional string

        The region of the user.

      • timezone: optional string

        The IANA timezone of the user.

  • WebFetchTool20250910 object { name, type, allowed_callers, 8 more }

    • name: "web_fetch"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "web_fetch"

    • type: "web_fetch_20250910"

      • "web_fetch_20250910"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • allowed_domains: optional array of string

      List of domains to allow fetching from

    • blocked_domains: optional array of string

      List of domains to block fetching from

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • citations: optional CitationsConfigParam

      Citations configuration for fetched documents. Citations are disabled by default.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • max_content_tokens: optional number

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    • max_uses: optional number

      Maximum number of times the tool can be used in the API request.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • WebSearchTool20260209 object { name, type, allowed_callers, 7 more }

    • name: "web_search"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "web_search"

    • type: "web_search_20260209"

      • "web_search_20260209"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • allowed_domains: optional array of string

      If provided, only these domains will be included in results. Cannot be used alongside blocked_domains.

    • blocked_domains: optional array of string

      If provided, these domains will never appear in results. Cannot be used alongside allowed_domains.

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • max_uses: optional number

      Maximum number of times the tool can be used in the API request.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

    • user_location: optional UserLocation

      Parameters for the user's location. Used to provide more relevant search results.

  • WebFetchTool20260209 object { name, type, allowed_callers, 8 more }

    • name: "web_fetch"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "web_fetch"

    • type: "web_fetch_20260209"

      • "web_fetch_20260209"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • allowed_domains: optional array of string

      List of domains to allow fetching from

    • blocked_domains: optional array of string

      List of domains to block fetching from

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • citations: optional CitationsConfigParam

      Citations configuration for fetched documents. Citations are disabled by default.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • max_content_tokens: optional number

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    • max_uses: optional number

      Maximum number of times the tool can be used in the API request.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • WebFetchTool20260309 object { name, type, allowed_callers, 9 more }

    Web fetch tool with use_cache parameter for bypassing cached content.

    • name: "web_fetch"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "web_fetch"

    • type: "web_fetch_20260309"

      • "web_fetch_20260309"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • allowed_domains: optional array of string

      List of domains to allow fetching from

    • blocked_domains: optional array of string

      List of domains to block fetching from

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • citations: optional CitationsConfigParam

      Citations configuration for fetched documents. Citations are disabled by default.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • max_content_tokens: optional number

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    • max_uses: optional number

      Maximum number of times the tool can be used in the API request.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

    • use_cache: optional boolean

      Whether to use cached content. Set to false to bypass the cache and fetch fresh content. Only set to false when the user explicitly requests fresh content or when fetching rapidly-changing sources.

  • ToolSearchToolBm25_20251119 object { name, type, allowed_callers, 3 more }

    • name: "tool_search_tool_bm25"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "tool_search_tool_bm25"

    • type: "tool_search_tool_bm25_20251119" or "tool_search_tool_bm25"

      • "tool_search_tool_bm25_20251119"

      • "tool_search_tool_bm25"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

  • ToolSearchToolRegex20251119 object { name, type, allowed_callers, 3 more }

    • name: "tool_search_tool_regex"

      Name of the tool.

      This is how the tool will be called by the model and in tool_use blocks.

      • "tool_search_tool_regex"

    • type: "tool_search_tool_regex_20251119" or "tool_search_tool_regex"

      • "tool_search_tool_regex_20251119"

      • "tool_search_tool_regex"

    • allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"

      • "direct"

      • "code_execution_20250825"

      • "code_execution_20260120"

    • cache_control: optional CacheControlEphemeral

      Create a cache control breakpoint at this content block.

    • defer_loading: optional boolean

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    • strict: optional boolean

      When true, guarantees schema validation on tool names and inputs

• top_k: optional number

  Only sample from the top K options for each subsequent token.

  Used to remove "long tail" low probability responses. Learn more technical details here.

  Recommended for advanced use cases only.

• top_p: optional number

  Use nucleus sampling.

  In nucleus sampling, we compute the cumulative distribution over all the options for each subsequent token in decreasing probability order and cut it off once it reaches a particular probability specified by top_p.

  Recommended for advanced use cases only.

Returns

• Message object { id, container, content, 7 more }

  • id: string

    Unique object identifier.

    The format and length of IDs may change over time.

  • container: Container

    Information about the container used in the request (for the code execution tool)

    • id: string

      Identifier for the container used in this request

    • expires_at: string

      The time at which the container will expire.

  • content: array of ContentBlock

    Content generated by the model.

    This is an array of content blocks, each of which has a type that determines its shape.

    Example:

    [{"type": "text", "text": "Hi, I'm Claude."}]
    

    If the request input messages ended with an assistant turn, then the response content will continue directly from that last turn. You can use this to constrain the model's output.

    For example, if the input messages were:

    [
      {"role": "user", "content": "What's the Greek name for Sun? (A) Sol (B) Helios (C) Sun"},
      {"role": "assistant", "content": "The best answer is ("}
    ]
    

    Then the response content might be:

    [{"type": "text", "text": "B)"}]
    

    • TextBlock object { citations, text, type }

      • citations: array of TextCitation

        Citations supporting the text block.

        The type of citation returned will depend on the type of document being cited. Citing a PDF results in page_location, plain text results in char_location, and content document results in content_block_location.

        • CitationCharLocation object { cited_text, document_index, document_title, 4 more }

          • cited_text: string

          • document_index: number

          • document_title: string

          • end_char_index: number

          • file_id: string

          • start_char_index: number

          • type: "char_location"

            • "char_location"

        • CitationPageLocation object { cited_text, document_index, document_title, 4 more }

          • cited_text: string

          • document_index: number

          • document_title: string

          • end_page_number: number

          • file_id: string

          • start_page_number: number

          • type: "page_location"

            • "page_location"

        • CitationContentBlockLocation object { cited_text, document_index, document_title, 4 more }

          • cited_text: string

            The full text of the cited block range, concatenated.

            Always equals the contents of content[start_block_index:end_block_index] joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

          • document_index: number

          • document_title: string

          • end_block_index: number

            Exclusive 0-based end index of the cited block range in the source's content array.

            Always greater than start_block_index; a single-block citation has end_block_index = start_block_index + 1.

          • file_id: string

          • start_block_index: number

            0-based index of the first cited block in the source's content array.

          • type: "content_block_location"

            • "content_block_location"

        • CitationsWebSearchResultLocation object { cited_text, encrypted_index, title, 2 more }

          • cited_text: string

          • encrypted_index: string

          • title: string

          • type: "web_search_result_location"

            • "web_search_result_location"

          • url: string

        • CitationsSearchResultLocation object { cited_text, end_block_index, search_result_index, 4 more }

          • cited_text: string

            The full text of the cited block range, concatenated.

            Always equals the contents of content[start_block_index:end_block_index] joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

          • end_block_index: number

            Exclusive 0-based end index of the cited block range in the source's content array.

            Always greater than start_block_index; a single-block citation has end_block_index = start_block_index + 1.

          • search_result_index: number

            0-based index of the cited search result among all search_result content blocks in the request, in the order they appear across messages and tool results.

            Counted separately from document_index; server-side web search results are not included in this count.

          • source: string

          • start_block_index: number

            0-based index of the first cited block in the source's content array.

          • title: string

          • type: "search_result_location"

            • "search_result_location"

      • text: string

      • type: "text"

        • "text"

    • ThinkingBlock object { signature, thinking, type }

      • signature: string

      • thinking: string

      • type: "thinking"

        • "thinking"

    • RedactedThinkingBlock object { data, type }

      • data: string

      • type: "redacted_thinking"

        • "redacted_thinking"

    • ToolUseBlock object { id, caller, input, 2 more }

      • id: string

      • caller: DirectCaller or ServerToolCaller or ServerToolCaller20260120

        Tool invocation directly from the model.

        • DirectCaller object { type }

          Tool invocation directly from the model.

          • type: "direct"

            • "direct"

        • ServerToolCaller object { tool_id, type }

          Tool invocation generated by a server-side tool.

          • tool_id: string

          • type: "code_execution_20250825"

            • "code_execution_20250825"

        • ServerToolCaller20260120 object { tool_id, type }

          • tool_id: string

          • type: "code_execution_20260120"

            • "code_execution_20260120"

      • input: map[unknown]

      • name: string

      • type: "tool_use"

        • "tool_use"

    • ServerToolUseBlock object { id, caller, input, 2 more }

      • id: string

      • caller: DirectCaller or ServerToolCaller or ServerToolCaller20260120

        Tool invocation directly from the model.

        • DirectCaller object { type }

          Tool invocation directly from the model.

        • ServerToolCaller object { tool_id, type }

          Tool invocation generated by a server-side tool.

        • ServerToolCaller20260120 object { tool_id, type }

      • input: map[unknown]

      • name: "web_search" or "web_fetch" or "code_execution" or 4 more

        • "web_search"

        • "web_fetch"

        • "code_execution"

        • "bash_code_execution"

        • "text_editor_code_execution"

        • "tool_search_tool_regex"

        • "tool_search_tool_bm25"

      • type: "server_tool_use"

        • "server_tool_use"

    • WebSearchToolResultBlock object { caller, content, tool_use_id, type }

      • caller: DirectCaller or ServerToolCaller or ServerToolCaller20260120

        Tool invocation directly from the model.

        • DirectCaller object { type }

          Tool invocation directly from the model.

        • ServerToolCaller object { tool_id, type }

          Tool invocation generated by a server-side tool.

        • ServerToolCaller20260120 object { tool_id, type }

      • content: WebSearchToolResultBlockContent

        • WebSearchToolResultError object { error_code, type }

          • error_code: WebSearchToolResultErrorCode

            • "invalid_tool_input"

            • "unavailable"

            • "max_uses_exceeded"

            • "too_many_requests"

            • "query_too_long"

            • "request_too_large"

          • type: "web_search_tool_result_error"

            • "web_search_tool_result_error"

        • array of WebSearchResultBlock

          • encrypted_content: string

          • page_age: string

          • title: string

          • type: "web_search_result"

            • "web_search_result"

          • url: string

      • tool_use_id: string

      • type: "web_search_tool_result"

        • "web_search_tool_result"

    • WebFetchToolResultBlock object { caller, content, tool_use_id, type }

      • caller: DirectCaller or ServerToolCaller or ServerToolCaller20260120

        Tool invocation directly from the model.

        • DirectCaller object { type }

          Tool invocation directly from the model.

        • ServerToolCaller object { tool_id, type }

          Tool invocation generated by a server-side tool.

        • ServerToolCaller20260120 object { tool_id, type }

      • content: WebFetchToolResultErrorBlock or WebFetchBlock

        • WebFetchToolResultErrorBlock object { error_code, type }

          • error_code: WebFetchToolResultErrorCode

            • "invalid_tool_input"

            • "url_too_long"

            • "url_not_allowed"

            • "url_not_accessible"

            • "unsupported_content_type"

            • "too_many_requests"

            • "max_uses_exceeded"

            • "unavailable"

          • type: "web_fetch_tool_result_error"

            • "web_fetch_tool_result_error"

        • WebFetchBlock object { content, retrieved_at, type, url }

          • content: DocumentBlock

            • citations: CitationsConfig

              Citation configuration for the document

              • enabled: boolean

            • source: Base64PDFSource or PlainTextSource

              • Base64PDFSource object { data, media_type, type }

                • data: string

                • media_type: "application/pdf"

                  • "application/pdf"

                • type: "base64"

                  • "base64"

              • PlainTextSource object { data, media_type, type }

                • data: string

                • media_type: "text/plain"

                  • "text/plain"

                • type: "text"

                  • "text"

            • title: string

              The title of the document

            • type: "document"

              • "document"

          • retrieved_at: string

            ISO 8601 timestamp when the content was retrieved

          • type: "web_fetch_result"

            • "web_fetch_result"

          • url: string

            Fetched content URL

      • tool_use_id: string

      • type: "web_fetch_tool_result"

        • "web_fetch_tool_result"

    • CodeExecutionToolResultBlock object { content, tool_use_id, type }

      • content: CodeExecutionToolResultBlockContent

        Code execution result with encrypted stdout for PFC + web_search results.

        • CodeExecutionToolResultError object { error_code, type }

          • error_code: CodeExecutionToolResultErrorCode

            • "invalid_tool_input"

            • "unavailable"

            • "too_many_requests"

            • "execution_time_exceeded"

          • type: "code_execution_tool_result_error"

            • "code_execution_tool_result_error"

        • CodeExecutionResultBlock object { content, return_code, stderr, 2 more }

          • content: array of CodeExecutionOutputBlock

            • file_id: string

            • type: "code_execution_output"

              • "code_execution_output"

          • return_code: number

          • stderr: string

          • stdout: string

          • type: "code_execution_result"

            • "code_execution_result"

        • EncryptedCodeExecutionResultBlock object { content, encrypted_stdout, return_code, 2 more }

          Code execution result with encrypted stdout for PFC + web_search results.

          • content: array of CodeExecutionOutputBlock

            • file_id: string

            • type: "code_execution_output"

          • encrypted_stdout: string

          • return_code: number

          • stderr: string

          • type: "encrypted_code_execution_result"

            • "encrypted_code_execution_result"

      • tool_use_id: string

      • type: "code_execution_tool_result"

        • "code_execution_tool_result"

    • BashCodeExecutionToolResultBlock object { content, tool_use_id, type }

      • content: BashCodeExecutionToolResultError or BashCodeExecutionResultBlock

        • BashCodeExecutionToolResultError object { error_code, type }

          • error_code: BashCodeExecutionToolResultErrorCode

            • "invalid_tool_input"

            • "unavailable"

            • "too_many_requests"

            • "execution_time_exceeded"

            • "output_file_too_large"

          • type: "bash_code_execution_tool_result_error"

            • "bash_code_execution_tool_result_error"

        • BashCodeExecutionResultBlock object { content, return_code, stderr, 2 more }

          • content: array of BashCodeExecutionOutputBlock

            • file_id: string

            • type: "bash_code_execution_output"

              • "bash_code_execution_output"

          • return_code: number

          • stderr: string

          • stdout: string

          • type: "bash_code_execution_result"

            • "bash_code_execution_result"

      • tool_use_id: string

      • type: "bash_code_execution_tool_result"

        • "bash_code_execution_tool_result"

    • TextEditorCodeExecutionToolResultBlock object { content, tool_use_id, type }

      • content: TextEditorCodeExecutionToolResultError or TextEditorCodeExecutionViewResultBlock or TextEditorCodeExecutionCreateResultBlock or TextEditorCodeExecutionStrReplaceResultBlock

        • TextEditorCodeExecutionToolResultError object { error_code, error_message, type }

          • error_code: TextEditorCodeExecutionToolResultErrorCode

            • "invalid_tool_input"

            • "unavailable"

            • "too_many_requests"

            • "execution_time_exceeded"

            • "file_not_found"

          • error_message: string

          • type: "text_editor_code_execution_tool_result_error"

            • "text_editor_code_execution_tool_result_error"

        • TextEditorCodeExecutionViewResultBlock object { content, file_type, num_lines, 3 more }

          • content: string

          • file_type: "text" or "image" or "pdf"

            • "text"

            • "image"

            • "pdf"

          • num_lines: number

          • start_line: number

          • total_lines: number

          • type: "text_editor_code_execution_view_result"

            • "text_editor_code_execution_view_result"

        • TextEditorCodeExecutionCreateResultBlock object { is_file_update, type }

          • is_file_update: boolean

          • type: "text_editor_code_execution_create_result"

            • "text_editor_code_execution_create_result"

        • TextEditorCodeExecutionStrReplaceResultBlock object { lines, new_lines, new_start, 3 more }

          • lines: array of string

          • new_lines: number

          • new_start: number

          • old_lines: number

          • old_start: number

          • type: "text_editor_code_execution_str_replace_result"

            • "text_editor_code_execution_str_replace_result"

      • tool_use_id: string

      • type: "text_editor_code_execution_tool_result"

        • "text_editor_code_execution_tool_result"

    • ToolSearchToolResultBlock object { content, tool_use_id, type }

      • content: ToolSearchToolResultError or ToolSearchToolSearchResultBlock

        • ToolSearchToolResultError object { error_code, error_message, type }

          • error_code: ToolSearchToolResultErrorCode

            • "invalid_tool_input"

            • "unavailable"

            • "too_many_requests"

            • "execution_time_exceeded"

          • error_message: string

          • type: "tool_search_tool_result_error"

            • "tool_search_tool_result_error"

        • ToolSearchToolSearchResultBlock object { tool_references, type }

          • tool_references: array of ToolReferenceBlock

            • tool_name: string

            • type: "tool_reference"

              • "tool_reference"

          • type: "tool_search_tool_search_result"

            • "tool_search_tool_search_result"

      • tool_use_id: string

      • type: "tool_search_tool_result"

        • "tool_search_tool_result"

    • ContainerUploadBlock object { file_id, type }

      Response model for a file uploaded to the container.

      • file_id: string

      • type: "container_upload"

        • "container_upload"

  • model: Model

    The model that will complete your prompt.

    See models for additional details and options.

    • "claude-opus-4-7" or "claude-mythos-preview" or "claude-opus-4-6" or 14 more

      The model that will complete your prompt.

      See models for additional details and options.

      • "claude-opus-4-7"

        Frontier intelligence for long-running agents and coding

      • "claude-mythos-preview"

        New class of intelligence, strongest in coding and cybersecurity

      • "claude-opus-4-6"

        Frontier intelligence for long-running agents and coding

      • "claude-sonnet-4-6"

        Best combination of speed and intelligence

      • "claude-haiku-4-5"

        Fastest model with near-frontier intelligence

      • "claude-haiku-4-5-20251001"

        Fastest model with near-frontier intelligence

      • "claude-opus-4-5"

        Premium model combining maximum intelligence with practical performance

      • "claude-opus-4-5-20251101"

        Premium model combining maximum intelligence with practical performance

      • "claude-sonnet-4-5"

        High-performance model for agents and coding

      • "claude-sonnet-4-5-20250929"

        High-performance model for agents and coding

      • "claude-opus-4-1"

        Exceptional model for specialized complex tasks

      • "claude-opus-4-1-20250805"

        Exceptional model for specialized complex tasks

      • "claude-opus-4-0"

        Powerful model for complex tasks

      • "claude-opus-4-20250514"

        Powerful model for complex tasks

      • "claude-sonnet-4-0"

        High-performance model with extended thinking

      • "claude-sonnet-4-20250514"

        High-performance model with extended thinking

      • "claude-3-haiku-20240307"

        Fast and cost-effective model

    • string

  • role: "assistant"

    Conversational role of the generated message.

    This will always be "assistant".

    • "assistant"

  • stop_details: RefusalStopDetails

    Structured information about a refusal.

    • category: "cyber" or "bio"

      The policy category that triggered the refusal.

      null when the refusal doesn't map to a named category.

      • "cyber"

      • "bio"

    • explanation: string

      Human-readable explanation of the refusal.

      This text is not guaranteed to be stable. null when no explanation is available for the category.

    • type: "refusal"

      • "refusal"

  • stop_reason: StopReason

    The reason that we stopped.

    This may be one the following values:

    • "end_turn": the model reached a natural stopping point
    • "max_tokens": we exceeded the requested max_tokens or the model's maximum
    • "stop_sequence": one of your provided custom stop_sequences was generated
    • "tool_use": the model invoked one or more tools
    • "pause_turn": we paused a long-running turn. You may provide the response back as-is in a subsequent request to let the model continue.
    • "refusal": when streaming classifiers intervene to handle potential policy violations

    In non-streaming mode this value is always non-null. In streaming mode, it is null in the message_start event and non-null otherwise.

    • "end_turn"

    • "max_tokens"

    • "stop_sequence"

    • "tool_use"

    • "pause_turn"

    • "refusal"

  • stop_sequence: string

    Which custom stop sequence was generated, if any.

    This value will be a non-null string if one of your custom stop sequences was generated.

  • type: "message"

    Object type.

    For Messages, this is always "message".

    • "message"

  • usage: Usage

    Billing and rate-limit usage.

    Anthropic's API bills and rate-limits by token counts, as tokens represent the underlying cost to our systems.

    Under the hood, the API transforms requests into a format suitable for the model. The model's output then goes through a parsing stage before becoming an API response. As a result, the token counts in usage will not match one-to-one with the exact visible content of an API request or response.

    For example, output_tokens will be non-zero, even for an empty string response from Claude.

    Total input tokens in a request is the summation of input_tokens, cache_creation_input_tokens, and cache_read_input_tokens.

    • cache_creation: CacheCreation

      Breakdown of cached tokens by TTL

      • ephemeral_1h_input_tokens: number

        The number of input tokens used to create the 1 hour cache entry.

      • ephemeral_5m_input_tokens: number

        The number of input tokens used to create the 5 minute cache entry.

    • cache_creation_input_tokens: number

      The number of input tokens used to create the cache entry.

    • cache_read_input_tokens: number

      The number of input tokens read from the cache.

    • inference_geo: string

      The geographic region where inference was performed for this request.

    • input_tokens: number

      The number of input tokens which were used.

    • output_tokens: number

      The number of output tokens which were used.

    • server_tool_use: ServerToolUsage

      The number of server tool requests.

      • web_fetch_requests: number

        The number of web fetch tool requests.

      • web_search_requests: number

        The number of web search tool requests.

    • service_tier: "standard" or "priority" or "batch"

      If the request used the priority, standard, or batch tier.

      • "standard"

      • "priority"

      • "batch"

Example

curl https://api.anthropic.com/v1/messages \
    -H 'Content-Type: application/json' \
    -H 'anthropic-version: 2023-06-01' \
    -H "X-Api-Key: $ANTHROPIC_API_KEY" \
    --max-time 600 \
    -d "{
          \"max_tokens\": 1024,
          \"messages\": [
            {
              \"content\": \"Hello, world\",
              \"role\": \"user\"
            }
          ],
          \"model\": \"claude-opus-4-6\",
          \"system\": [
            {
              \"text\": \"Today's date is 2024-06-01.\",
              \"type\": \"text\"
            }
          ],
          \"temperature\": 1,
          \"thinking\": {
            \"type\": \"adaptive\"
          },
          \"tools\": [
            {
              \"input_schema\": {
                \"type\": \"object\",
                \"properties\": {
                  \"location\": \"bar\",
                  \"unit\": \"bar\"
                },
                \"required\": [
                  \"location\"
                ]
              },
              \"name\": \"name\"
            }
          ],
          \"top_k\": 5,
          \"top_p\": 0.7
        }"

Response

{
  "id": "msg_013Zva2CMHLNnXjNJJKqJ2EF",
  "container": {
    "id": "id",
    "expires_at": "2019-12-27T18:11:19.117Z"
  },
  "content": [
    {
      "citations": [
        {
          "cited_text": "cited_text",
          "document_index": 0,
          "document_title": "document_title",
          "end_char_index": 0,
          "file_id": "file_id",
          "start_char_index": 0,
          "type": "char_location"
        }
      ],
      "text": "Hi! My name is Claude.",
      "type": "text"
    }
  ],
  "model": "claude-opus-4-6",
  "role": "assistant",
  "stop_details": {
    "category": "cyber",
    "explanation": "explanation",
    "type": "refusal"
  },
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "type": "message",
  "usage": {
    "cache_creation": {
      "ephemeral_1h_input_tokens": 0,
      "ephemeral_5m_input_tokens": 0
    },
    "cache_creation_input_tokens": 2051,
    "cache_read_input_tokens": 2051,
    "inference_geo": "inference_geo",
    "input_tokens": 2095,
    "output_tokens": 503,
    "server_tool_use": {
      "web_fetch_requests": 2,
      "web_search_requests": 0
    },
    "service_tier": "standard"
  }
}